The invention relates to a method for monitoring slide bearings of a crank mechanism of a piston engine with at least one cylinder whose piston is drivably connected with a crankshaft via a connecting rod which is loaded alternatingly under tension and under pressure by the piston in the zone of a dead center of the crank mechanism depending on the cycle of the piston engine, with an operating parameter which changes on the occurrence of bearing damage being repeatedly measured and evaluated.
Since generally any damage to slide bearings begins to show by a temperature increase in the zone of the bearing surface, it has already been proposed (EP 0161644 A2, DD 249075 A1, DE 19614803 A, JP 61105306 A) to monitor the temperature in the zone of the bearing shell during the bearing operation in order to recognize any bearing damage announced by an increase in the temperature at an early time and to enable precautions to be taken for respective countermeasures. For this purpose temperature sensors are inserted into bores of the bearing supporting body, which bores extend in part up to the bearing shell. The disadvantage in these known apparatuses for monitoring the temperature in the running layer zone is on the one hand that the bearing shells need to be tapped in order to avoid having to cope with longer response times for temperature detection which are caused by heat conduction. Despite tapping the bearing shell at several places the short response times necessary for the timely recognition of damage to the bearing is exceeded in such cases when the damage which announces itself by an increase in the temperature is situated outside of the direct measurement zones which are determined by the position of the bores for the temperature sensors. This means that the local increases in temperature in the zone of the bearing layer of a slide bearing cannot be detected in a timely fashion with the reliability demanded for the timely recognition of damage to the bearing. Similar disadvantages occur when it is not the temperature in the zone of the bearing layer but instead the lubricant pressure in the bearing clearance that is detected by way of a bore which penetrates the bearing box (JP 11072398 A) or when additional temperature sensors are provided in the zone of the measuring location for the lubricant pressure (EP 0029736 A1, JP 5782742A).
The invention is thus based on the object of monitoring the slide bearings of a crank mechanism of a piston engine with the help of a method of the kind mentioned above in such a way that in order to detect the operating parameters which change with the occurrence of a damage to the bearing it is necessary to tap neither the supporting body nor the bearing boxes of the slide bearings. Furthermore, any changes of the operating parameters which announce a damage to the bearing are to be recognized as early as possible.
The invention achieves the object in accordance with the invention in such a way that the position of at least one part of the crank mechanism, preferably the piston position, is measured at the dead center of the crank mechanism under pressure and tensile load of the connecting rod and the difference between the positions for the pressure and tensile load is evaluated.
The invention is based on the finding that the bearing surface is deformed within the terms of an increase of the bearing clearance on the one hand by the rise in temperature which occurs by the commencing fatigue of the bearing layer material and is caused by increasing metallic contacts between bearing and shaft as a result of fatigue cracks causing local deformations of the bearing surface and outbreaks of the bearing layer and on the other hand by the dynamic load. Through the progressively increasing bearing clearance it is thus possible to monitor the slide bearings with respect to an early recognition of damage under the precondition that changes of the bearing clearance can be determined in a sufficiently precise way. This is achieved in accordance with the invention in crank mechanisms of piston engines in such a way that the piston position is measured at a dead center of the crank mechanism on the one hand under the pressure load of the connecting rod and on the other hand under the tensile load of the connecting rod. If the connecting rod, in accordance with the cycle of the piston engine, is subjected in the dead center zone of the crank mechanism to a tensile load by the piston, the piston position is determined by the bearing clearances in the direction of the tensile load in addition to the occurring elastic extensions of the parts of the crank mechanism. In the case of a pressure load of the connecting rod via the piston there is an opposite limit stop of the parts at the illustrated dead center of the crank mechanism in the zone of the bearing clearances, so that the piston positions differ by the sum total of the bearing clearances on the one hand for the tensile load and on the other hand for the pressure load of the connecting rod in the illustrated dead center of the crank mechanism, which is apart from the also opposite elastic deformations of the involved parts, which allows a sufficiently precise monitoring of changes in the bearing clearance for the early recognition of damage to the bearing. Since merely the difference of the piston positions is detected, any heat-induced expansions of the parts of the crank mechanism have no influence on the precision of the detection of the bearing clearance. It is understood that also the connecting rod position or the position of the crankshaft can be detected in the respective dead center of the crank mechanism when only the bearings of the con-rod head and the crankshaft or only the crankshaft bearings are to be monitored.
The measurement of the piston positions or the positions of the respectively detected part of the crank mechanism at the dead center of the crank mechanism is preferably made in a contactless fashion. For the purpose of determining the position (like the detection of the position for other parts of the crank mechanism), different reference variables can be used because merely the position with respect to the cylinder or the crankcase is relevant. Particularly simple constructional conditions are obtained, however, when the face side of the piston on the con-rod side is used.